1. Field
Embodiments of the disclosure relate generally to the field of control surface actuation systems and more particularly to embodiments for a shape memory alloy actuator with dual collinear shape memory alloy tubes having antagonistic reaction to form a smart spring return with a torque sensor.
2. Background
Wind tunnel models typically require movable control surfaces to allow simulation of various control aspects of the vehicle being modeled. Unmotorized surfaces are often used due to their simplicity. However, such surfaces must be positioned by hand requiring interruption of testing to position the surfaces at desired control angles. Models are typically of reduced scale and therefore full size actuators which would be employed in actual vehicles are not readily adaptable for use. Various actuation systems have been employed in wind tunnel models including electromechanical actuators and shape memory alloy (SMA) actuators using wires for hinge moment actuation using differential pull from SMA wires. However, electromechanical actuation is relatively bulky because of low power densities and the need for complex electric motor/gear assemblies. As such, the amount of space required in the supporting structure (for example in a vertical tail) may limit the amount of instrumentation such as pressure sensors that can be installed in the model and may reduce the structural strength which tends to limit their use to lower pressure tunnels having lower loads. Lower pressure tunnels do not match the aerodynamic characteristics of a full scale airplane as well which limits their fidelity as design tools for testing aircraft configurations. SMA wire actuation has limited power and strength, and therefore is similarly suitable for low pressure wind tunnel testing only.
Current shape memory alloys, SMA, such as Nitinol have high force output while transitioning from martensite to austenite or low to high temperatures. The austenite to martensite transition will output significantly lower forces even for a well trained actuator. Most actuator designs ignore the force generated in the actuator as the material transitions from austenite to martensite or assume the useable force to be very small. Actuator designs commonly use a return spring to apply a force opposing the force generated by the SMA during transition from martensite to austenite. An appropriately sized return spring will allow greater displacement of the actuator or higher recoverable strain in the SMA. The tradeoff is that the useable force output of the SMA is decreased because the return spring must be accounted for in the total output of the actuator. A conventional spring that is capable of applying a given load at the actuator stowed position will apply a greater return force opposite the SMA as the actuator deploys following Hooks law. Ideally a spring with decreasing spring rate would be very well suited for an actuator such that a high spring load is only seen at the retracted or nominal actuator position. In practice a passive spring load with decreasing load is not trivial but can be accomplished using cams, linkages or complex spring geometry. Frequently a spring solution becomes heavier and requires more volume than the SMA actuator reducing the most desirable characteristics of SMA actuators.
It is therefore desirable to provide actuators for use in space constrained applications with non-linear return spring characteristics. It is also desirable to provide an actuator having power density much greater than traditional solutions for actuated control surfaces allowing its implementation into applications requiring higher forces and into more restrictive spaces. It is further desirable to provide an actuator with the ability to place the actuating elements on the hinge line of a control surface.